Stabilization of fluorescent dyes in vinyl articles using hindered amine light stabilizers

ABSTRACT

A durable colored article having fluorescent properties comprises a substantially solventless polyvinyl chloride matrix, a thioxanthene fluorescent dye, and a secondary or tertiary hindered amine light stabilizer having a molecular weight less than about 1000 grams/mole. The invention has the advantage in that it provides a flexible polyvinyl chloride film that exhibits durable fluorescent colors.

TECHNICAL FIELD

The present invention pertains to polyvinyl chloride articles thatexhibit durable fluorescent colors through the use of selected hinderedamine light stabilizers.

BACKGROUND

Articles containing colorants lose their color when exposed to solarradiation for extended time periods. For example, articles placedoutdoors throughout the summer often tend to display a faded version oftheir original color by the time autumn arrives. Although this fadingoccurs in both conventional and fluorescent colorants, the problem ismore acute with fluorescent colorants.

The life of fluorescent colored articles is typically in the range ofmonths when exposed to daily solar radiation, whereas the life ofarticles that use conventional colorants can be in the range of years.Although generally less stable, fluorescent colorants nonetheless findfrequent use because of their ability to increase an article'svisibility. Unlike conventional colorants, fluorescent colorants cantake light that they absorb and reemit it in the visible spectrum. Thisinnate property allows fluorescent articles to exhibit an enhancedvisual contrast between the colored article and its surroundingenvironment.

Investigators in the retroreflective art have attempted to stabilizepolymeric articles containing fluorescent colorants using various means.For example, Burns et al. in U.S. Pat. No. 5,605,761 teach the use of ahindered amine light stabilizer (HALS) to maintain the durability ofarticles containing fluorescent dyes in a 30 polycarbonate polymericmatrix. The document further discloses that the fluorescent dye may bethioxanthene, perylene imide, or thioindigoid dyes, and the HALS may becompounds from the 2,2,6,6-tetraalkyl piperidine class of compounds.While these articles are extremely useful in maintaining fluorescentcolor stability, they are not very flexible due to the polycarbonatematrix's inherent rigidity.

Others, such as Pavelka et al. in U.S. Pat. No. 5,387,458 have attemptedto maintain fluorescent colors by using an ultraviolet screening layerthat screens out ultraviolet (UV) radiation in the range of 340 to 400nanometers. The document also discloses that the fluorescent colorresides in a separate layer rather than in the screening layer. Althoughthese articles are highly beneficial because of their stable fluorescentcolors, they do present the need of having two separate layers that canadd cost to the construction. Furthermore, the screening layer may notbe effective in reducing the degradation of the fluorescent dye causedby dye absorbtion of visible radiation.

Polyvinyl chloride (PVC) films are useful in many applications becauseof their flexibility and commercial availability. UV absorbingstabilizers have been commonly used in polyvinyl chloride articles tolight stabilize the polymer matrix. See, e.g., Marice McMurrer, Update:UV Stabilizers, PLASTICS COMPOUNDING 40 (January/February 1985). UVstabilizers, however, are not effective in stabilizing fluorescent dyesin the matrix.

Although PVC films containing fluorescent dyes are widely availabletoday, they tend to have very poor color retention. Factors contributingto the color fading include lack of dye solubility in the PVC hostmatrix, dye migration, and minimal protection offered by the resinagainst photodegradation.

Technical publications have suggested that HALS, with its amine group inthe molecular structure, may not be compatible with PVC. For example, T.Hjertberg and E. M. Sörvik stated in Thermal Degradation of PVC, inDEGRADATION AND STABILISATION OF PVC, E. D. Owen (editor) 21, 69 (1984)that amines “induce dehydrochlorination of PVC at high temperatures”leading to degradation of the PVC matrix. In addition, HALS based onsecondary or tertiary piperidinyl amines are very basic compounds. Forexample, 2,2,6,6-tertamethyl piperidine has a pk_(b) of 2.9 as comparedto 4,7 for ammonia when measured in water. See Can Zhang et al.,Hindered Amine Light Stabilizers: Effects of Acid Exposure, Volume 24 ofJOURNAL OF POLYMER SCIENCE: PART C: POLYMER LETTERS 453, 453 (1986).Because of its alkalinity, HALS in the presence of a volatile acid, suchas hydrochloric acid (HCl), forms a salt. Hydrochloric acid is producedby degradation and oxidation reactions resulting from “light inducedaging of PVC films.” See Martinez et al., Prediction of PhotoageingStability of Plasticized PVC Films Containing UV-Stabilisers, Volume 54of POLYMER DEGRADATION AND STABILITY 49, 49 (1996). The presence of abasic HALS in combination with a readily available source of HCl givesrise to acid-base reactions that can degrade the PVC matrix.

Because of the flexible nature of PVC films and the desirability usingof fluorescent colorants in many articles, there is a need for a durablecolored article having these combinations.

SUMMARY OF THE INVENTION

The present invention provides, for the first time, colored articlesexhibiting durable fluorescent properties in a solventless PVC hostmatrix by incorporating a particular class of HALS to stabilize a classof fluorescent dyes. Contrary to known teachings that HALS may not becompatible with PVC, this invention includes the discovery that newcombinations of HALS and fluorescent dyes in a PVC host matrix willexhibit superior stabilization of colored, fluorescent articles. Becausethe PVC host matrix has good mechanical and thermal properties, theinventive article will be useful in many applications, including, butnot limited to, uses in clothing, traffic control signs and devices (forexample, roll-up signs), backpacks, and water flotation safety devices.

In brief summary, the inventive article exhibits durable color andfluorescent properties and comprises (a) a polymeric matrix thatcontains substantially solventless polyvinyl chloride resin; (b) athioxanthene fluorescent dye; and (c) a hindered amine light stabilizercomprising at least one secondary or tertiary amine groups and having amolecular weight of less than about 1000 grams/mole. The inventivearticles can be made by combining these components into a mixture andforming an article from the mixture.

Because processing of a substantially solventless polyvinylchlorideresin subjects the resin to high temperatures, it was not predicted thata durable fluorescent-colored article would result. As indicated above,amines can induce dehydrochlorination of the polyvinyl chloride at hightemperatures, which can lead to degradation of the polyvinyl chloridematrix. Notwithstanding this concept, the inventive article issurprisingly durable. Thus, the combination of using substantiallysolventless polyvinyl chloride and HALS provides benefits unsuggested inthe art for forming durable fluorescent-colored, PVC articles.

The present invention has the advantage in that it exhibits durablecolor properties and fluorescence without the need to use protectiveoverlays. If desired, however, a protective overlay may be used tofurther increase the durability of the inventive article. The inventivearticles retain their color and are able to fluoresce for a longer timeperiod than is normally expected even when they are exposed to directsunlight. Articles of the invention therefore are good candidates foruse with retroreflective elements.

Another advantage of the invention is that the polymers, dyes, and HALSmay be processed in a solventless system, which not only essentiallyeliminates solvent emissions into the atmosphere but also reduces thearticle's manufacturing cost by totally eliminating solvent use.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further explained with reference to the drawings,wherein:

FIG. 1 is a cross-sectional view of a retroreflective article 10 inaccordance with the invention;

FIG. 2 is a cross-sectional view of another embodiment ofretroreflective article 20 in accordance with the invention;

FIG. 3 is a cross-sectional view of another embodiment ofretroreflective article 30 in accordance with the invention;

FIG. 4 is a cross-sectional view of another embodiment ofretroreflective article 50 in accordance with the invention; and

FIG. 5 is a cross-sectional view of another embodiment ofretroreflective article 70 in accordance with the invention.

These figures are idealized, are not to scale, and are intended to bemerely illustrative and non-limiting.

DEFINITIONS

As used herein:

“colorant” means pigments or dyes or other substances used to impart hueand chroma to an article;

“conventional colorant” means colorants that do not significantlyfluoresce when exposed to visible light and/or ultraviolet light and donot exhibit fluorescent properties to the unaided eye;

“cube film” means a single retroreflective film having cube cornerelements projecting from one surface thereof,

“cube corner sheeting” means a multilayer retroreflective sheeting thatcontains cube corner elements;

“durable” refers to an enhanced retention of color or fluorescence uponexposure to weathering;

“embedded lens” retroreflective base sheet comprises (a) a monolayer ofmicrospheres having a space layer and (b) a reflective layer in opticalassociation with the rear surface of the microspheres and a binder layerin which the front surfaces of the microspheres are embedded;

“encapsulated lens” retroreflective base sheet comprises (a) a monolayerof microspheres having a reflective layer in association with the rearsurface of the microspheres and (b) a cover layer disposed over thefront surface of the microspheres forming cells;

“exposed lens” retroreflective base sheet comprises a monolayer ofmicrospheres having a reflective layer in association with the rearsurface of microspheres that are embedded in a binder layer;

“hindered amine light stabilizer” means an additive used to lightstabilize fluorescent dyes, the stabilizer having at least one secondaryor tertiary amine group;

“polymeric matrix” means the principal polymeric material in which thefluorescent dye and hindered amine light stabilizer reside;

“secondary amine group” means a group that contains nitrogen (N) and hasone hydrogen (H) atom bonded to the nitrogen atom;

“tertiary amine group” means a group that contains nitrogen (N) and doesnot have a hydrogen (H) atom bonded to the nitrogen atom;

“substantially solventless polyvinyl chloride resin” means a polymericpolyvinyl chloride resin capable of being processed, whether throughextrusion or calendering, without the use of a solvent;

“thioxanthene fluorescent dye” means a fluorescent dye having athioxanthene unit as part of its molecular structure;

“weathering” means exposing an article to either natural or artificialenvironments including, for example, heat, light, moisture, andultraviolet radiation.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

The present invention combines a substantially solventless polyvinylchloride host matrix with fluorescent dyes, and hindered amine lightstabilizers to yield durable, colored fluorescent articles.

FIG. 1 shows a cube corner based retroreflective article 10 of theinvention. Article 10 (commonly referred to as “cube film”) comprises amultitude of cube corner elements 12 and a land layer 14. Not shown inthe figure are fluorescent dyes and hindered amine light stabilizers.Light enters the cube film 10 through the front or first major surface15. The light then passes through the land layer 14 and strikes theplanar faces 11 of the cube corner elements 12 and returns in thedirection from which it came as shown by arrow 18.

FIG. 2 shows a cube corner based retroreflective article 20 of theinvention Article 20 comprises a body layer 26 disposed on a front orfirst major surface 25 of a cube film 21. The cube film 21 comprises amultitude of cube corner elements 22 and can optionally include a landlayer 24. In a preferred embodiment, the body layer 26 comprises asubstantially solventless polyvinyl chloride matrix, fluorescent dyes,and hindered amine light stabilizers (all not shown) and is theoutermost layer of article 20. The land layer 24 is distinguished fromthe body layer 26 by being a layer disposed immediately adjacent to thebase of the cube corner elements. If desired, the land layer 24, ifpresent, and/or the cube corner elements 22 can comprise a substantiallysolventless polyvinyl chloride matrix, fluorescent dyes and hinderedamine light stabilizers.

FIG. 3 shows a microsphere based retroreflective article 30 of theinvention. Article 30 comprises a body layer 36 disposed on the front orfirst major surface 35 of an embedded lens retroreflective base sheet31. For an illustrative example of an embedded lens sheet, see U.S. Pat.No. 4,505,967 (Bailey). Base sheet 31 comprises a monolayer ofmicrospheres 32 embedded in a binder layer 33 with space layer 34,specular reflective layer 38 and optional adhesive layer 40. Lightenters retroreflective article 30 through its front surface 41. Thelight then passes through the body layer 36 and the binder layer 33,strikes microspheres 32, passes through space layer 34 to strike thespecular reflective layer 38, and returns in the direction from which itcame as shown by arrow 37.

The retroreflective base sheet can also be exposed lens or encapsulatedlens—see U.S. Pat. No. 5,316,838 (Crandall) and U.S. Pat. No. 4,025,159(McGrath) respectively for examples of such sheeting. In a preferredembodiment, the body layer 36 comprises a substantially solventlesspolyvinyl chloride matrix, fluorescent dyes, and hindered amine lightstabilizers.

Although not necessary, articles of the invention may optionally includea protective overlay that may or may not include ultraviolet absorbingagents. The overlay is preferably substantially transparent to visiblelight and includes a means to screen substantial portions of incidentultraviolet radiation, FIG. 4 illustrates a retroreflective embodiment50 having a cube film 51. Body layer 56 is disposed on the front orfirst major surface 55 of cube film 51. Disposed on a first side 57 ofbody layer 56 is an overlay 58. In a preferred embodiment, body layer 56comprises a substantially solventless polyvinyl chloride matrix,fluorescent dyes, and hindered amine light stabilizers. Overlay 58 ispreferably coextensive with body layer 56 so as to provide the mostprotection.

The polymeric matrix used in the present invention containssubstantially solventless polyvinyl chloride as the host matrix. Thepolymeric matrix does not need to possess other polymers (e.g., acrylicpolymers) to impart good durability and thus may consist essentially ofsolventless polyvinyl chloride. Plasticizers may be incorporated intothe matrix to impart desirable physical properties, such as flexibility.Illustrative examples of useful plasticizers include di-2-ethylhexylphthalate, commercially available as DOP from Aristech Chemical Corp.,and diisononyl phthalate, commercially available as JAYFLEX DINP, fromExon Corp UV absorbers such as hydroxybenzophenones can be added tostabilize the PVC from ultraviolet light degradation. Other additivesthat may be added as processing aids include fillers, heat stabilizers,and lubricants.

Plasticized PVC is advantageous in that it has excellent flexibility soas to be conformable to a variety of diverse substrates ranging fromfabrics to substrates with compound curves, such as a traffic barrel.Articles of the invention have sufficient flexibility to be wound atroom temperature about a mandrel, having a diameter of 3 millimeterwithout cracking. Plasticized PVC can also be attached easily to asubstrate, through adhesive means or mechanical means. An illustrativemechanical means involves sewing the inventive product onto a fabricsubstrate.

The substantially solventless PVC films may be made by extruding orcalendering PVC resins combined with fluorescent dyes and HALS into afilm or cube film having a nominal thickness of about 0.025 millimeters(mm) (0.001 inch) to about 3.2 mm (0.125 inch), preferably about 0.076mm (0.003 inch) to about 0.5 mm (0.02 inch). The latter range ispreferable in that it is more useful for retroreflective sheetings. Filmthickness may vary with the particular application. For example, if theapplication requires high durability, typically a thicker film, on theorder of about 0.75 mm (0.030 inch) may be more useful. The thickness ofthe PVC film or cube film has an affect on the quantity of fluorescentdyes and hindered amine light stabilizers that can be loaded into thefilm.

The fluorescent dyes useful for this invention are dyes from thethioxanthene classes of compounds. A single dye or a combination of dyesmay be used.

Illustrative commercially available thioxanthene fluorescent dyes usefulin the present invention include HOSTASOL® RED GG, HOSTASOL® YELLOW 3G,DAY-GLO® D-304, and DAY-GLO® D-315.

A useful fluorescent orange dye is14H-anthra[2,1,9-mna]thioxanthene-14-one, commercially available as C.I.Solvent Orange 63 (HOSTASOL® RED GG) from Hoescht Celanese, and havingthe following chemical structure:

A useful yellow fluorescent dye isN-octadecyl-benzo[k,1]thioxanthene-3,4-dicarboximide, commerciallyavailable as C.1. Solvent Yellow 98 (HOSTASOL® YELLOW 3 G) from HoeschtCelanese, and having the following chemical structure:

Another useful yellow fluorescent dye is DAY-glo D-304, which is athioxanthene compound, available from Day-GLO® Color Corp., Cleveland,Ohio. Another useful orange fluorescent dye is DAY-GLO® D-315, also athioxanthene compound available from Day-Glo Color Corp.

Typically, up to 2 weight percent and preferably about 0.01 weightpercent to about 1.0 weight percent of the dye is present in theinventive film. The weight percent is based on the total weight of theinventive film. Dye loadings outside this range may be used inaccordance with the invention to achieve the desired color. For example,it the dye is added to a thicker film, a lower dye loading can give thesame visual effect. Articles having higher dye loadings generallyexhibit brighter fluorescence and deeper color than articles with lowerdye loadings of the same dye. Articles having a high dye loading,however, may exhibit a self-quenching phenomenon that occurs whenmolecules of the dye absorb the energy emitted by neighboring dyemolecules. This self-quenching can cause an undesirable decrease influorescent brightness.

Articles that possess excess dye can become opaque—perhaps because someof the excess dye may have not dissolved into the polymeric matrix. Forapplications that require the inventive articles to be lighttransmissive, such as applications requiring retroreflection, personsskilled in the art should take care to select an appropriate dye loadingso that substantially all of the dye dissolves into the polymericmatrix. For applications that do not require light transmissivity, suchas decorative applications, the dye loading may not be as importantbecause opacity is not a problem.

Other dyes and pigments (whether fluorescent or non-fluorescent) may beadded to the present invention to adjust the color and appearance of thearticle. Care should be taken, however, to select dyes and pigments, aswell as their loadings, so as not to significantly interfere with theperformance of the fluorescent dyes in the article. If retroreflectiveelements are included in the inventive article, the dyes or pigmentsshould not undesirably impair the article's transparency. If theinventive article has reduced transparency, its retroreflectiveperformance may also be undesirably reduced.

As discussed, numerous technical articles have indicated that a hinderedamine light stabilizer (HALS), with its amine group, is not compatiblewith polyvinyl chlorides. Thus the use of certain HALS to lightstabilize the inventive fluorescent colored PVC articles is verysurprising.

Without intending to be bound by theory, it is believed that thecombination of selected HALS, the substantially solventless polyvinylchloride host matrix, and selected fluorescent dyes in the presentinvention prevents an as yet undefined degradation and/or reactionbetween the dye and the polyvinyl chloride which could otherwise occur.Insofar as we know, the advantages of the present invention are attainedthrough the combination of the substantially solventless polyvinylchloride matrix, the thioxanthene fluorescent dye, and the hinderedamine light stabilizers described herein.

Typically, up to about 2 weight percent, and preferably about 0.05 toabout 1.0 weight percent of the HALS is contained in the inventivearticle. The weight percent of HALS used is based on the total weight ofthe inventive film.

Illustrative commercially available HALS useful in the present inventioninclude TINUVIN® 770, TINUVIN® 144, and SANDUVOR® PR-31.

A HALS, having the chemical formula ofBis-(2,2,6,6-tetramethyl-4-piperidinyl)sebacate and a molecular weightof about 480 grams/mole, contains secondary amines, is commerciallyavailable as TINUVIN® 770 from Ciba-Geigy Corp, and has the followingchemical structure:

This HALS possesses two secondary amine groups, where the nitrogen atomis bonded to two carbon atoms and a hydrogen atom.

A HALS, having the chemical formula ofBis-(1,2,2,6,6-pentamethyl-4-piperidinyl)-2-n-butyl-2-(3,5-di-tert-butyl-4-hydroxybenzyl)malonateand a molecular weight of about 685 grams/mole, contains tertiaryamines, is commercially available as TINUVIN® 144 from Ciba-Geigy Corp.,and has the following chemical structure:

A HALS, having a chemical formula of propanedioicacid,[(4-methoxyphenyl)-methylene]-bis-(1,2,2,6,6-pentamethyl-4-piperidinyl)ester,and a molecular weight of about 529 grams/mole, contains tertiaryamines, is commercially available as SANDUVOR® PR-31 from ClariantCorp., and has the following chemical structure:

TINUVIN® 144 and SANDUVOR® PR-31 each possesses two tertiary aminegroups, where the nitrogen atom is bonded to three carbon atoms.

Method of Making

The inventive film can be made using an extrusion or a calenderingmethod. Although both methods are useful in producing a substantiallyflat film, they do so by different processes. Extrusion involvesprocessing a viscous melt under pressure to force it through a shapingdie in a continuous stream to form a film. Calendering takes a mass offused, viscous material and feeds it between successive pairs ofco-rotating, parallel rolls to form a film. Extrusion has the advantagein that if a cube film is desired, the feed stock leaving the extrudercan be nipped directly into a mold having cube corner recesses.Calendering, on the other hand, has the advantage in that flexible PVCfilms can be economically produced using this process.

A method of making an article exhibiting durable color and fluorescentproperties can comprise: (a) combining substantially solventlesspolyvinyl chloride resin, a thioxanthene fluorescent dye, and a hinderedamine light stabilizer comprising at least one secondary or tertiaryamine groups having a molecular weight less than 1000 grams/mole into amixture; and (b) forming the article from the mixture.

Typically, in an extrusion process the polymeric resin/dye/HALS mixtureis first tumble mixed together. The polymeric resin is typically in theform of small granules. The mixture is fed into an extruder where, withthe presence of heat and rotational action of the screw, the mixture ismixed and changes into a viscous melt. Typically, an extruder withmultiple zones of heating is used. The extrusion temperature should bechosen to melt the components but not be so high so as to degrade them.Suitable extrusion temperatures, when using the fluorescent dyes andHALS described above, range from about 175° C. to about 205° C.Typically, the melt leaving the extrusion dye is allowed to contact achrome roll or polished casting roll to form a substantially flat film.

If desired, the melt leaving the extrusion die is allowed to contact amold or tool having cube corner recesses therein. When the melt isnipped into the mold, a cube corner film is formed having, preferably, aminimal land layer and a multitude of cube corner elements whose baseplane is adjacent to the land layer. See, for example, U.S. Pat. No.5,450,235 (Smith et al.) and International Publication No. WO 95/11464(Benson et al.) for descriptions of methods of producing a cube cornersheeting. Extrusion is the preferred method for making an inventive cubefilm.

The cube corner elements may optionally be vapor coated with a metalliclayer, such as vapor deposited aluminum or silver, to increaseretroreflective performance. Vapor coating the cube corner elements,however, may cause the fluorescent cube film to have a gray appearance,which may be undesirable for some applications.

In a calendering process, the polyvinyl chloride resin (typically inpowder form), the fluorescent dye, and the hindered amine lightstabilizer are added to mixing unit for intensive mixing. Otheradditives, such as plasticizers, UV absorbers, heat stabilizers,fillers, and lubricants may be added for desired physical propertiesand/or as processing aids. Typically the mixing unit has a ribbon typeblade and can be jacketed for heating and cooling. During mixing, thePVC powder absorbs the additives, including the dye and HALS, to form apowder mix. After intensive mixing, the powder is typically cooled andfed through a screen to remove metals because the metal particles, ifpresent, can damage the calender roll surface. The screened powdermixture is typically fed into a fluxing unit for continuous mixingcausing the mixture to become a fused, viscous mass that is feed stockto be delivered to the calender rolls. The calender rolls, typically ina four roll setup, can be heated. In making the inventive article, thecalender rolls are heated so that their surface temperature ranges fromabout 170° C. to about 180° C. (340 to 355° F.), Configuration of therolls can also be an important factor. The viscous, fused feedstock isfed to the calender where the film or sheet is formed with the filmthickness controlled by the gap between the final rolls.

Although this sequence is typical for a calendering process, manyvariations are possible depending on the end product desired.Calendering is a preferred method for making the inventive film becauseof economic efficiencies.

Given what is known in the art about amines inducing dehydrochlorinationof PVC at high temperatures, the invention nonetheless discovered thatPVC articles produced from calendering or extrusion with temperatures ashigh as 205° (355° F.) are durable, as shown herein by the examples,

Substantially flat films, whether produced by extrusion or calendering,can be laminated to a preexisting retroreflective base sheet, such ascube corner based or microsphere based sheets. Typically, the film islaminated to the front or first major surface of retroreflective basesheets to produce a new retroreflective article in accordance with thepresent invention. For example, as shown in FIG. 2, the body layer 26,typically a substantially flat film, is laminated to a front or firstmajor surface 25 of cube film 21 to produce a retroreflective article 20of the invention. Similarly, in FIG. 3, the body layer 36, typically asubstantially flat film, is laminated to a front or first major surface35 of microsphere based retroreflective base sheet 31 to produce aretroreflective article 30 of the invention.

In a preferred embodiment, the inventive films are used as a carrier forradiation cured cube corner elements. These cube corner elementscomprise reactive resins capable of being crosslinked by a free radicalpolymerization mechanism by exposure to actinic radiation, for example,electron beam, ultraviolet light, or visible light. See U.S. Pat No.5,450,235 (Smith et al.) and International Publication No. WO 95/11464for examples of such reactive resins. The reactive resin is preferablycured in situ on the inventive film. FIG. 5 shows a cube corner basedretroreflective article 70 of the invention manufactured in accordancewith the principles of the invention disclosed in InternationalPublication No. WO 95/11464 published Apr. 27, 1995, entitled“Ultra-Flexible Retroreflective Cube Corner Composite Sheetings andMethods of Manufacture.” The embodiment in FIG. 5 is designed to be ahighly flexible retroreflective sheeting suitable for conforming tocorrugated and/or flexible surfaces.

As shown in FIG. 5, retroreflective article 70 comprises a multitude ofsubstantially independent cube corner elements 72 and a body layer 76having two major surfaces 71 and 73, the cube corner elements projectingfrom the first major surface 73 and have zero to minimal land. Thus,this embodiment has essentially no land layer and the front surface 75of the cube corner elements is juxtaposed against surface 73. In apreferred embodiment, body layer 76 comprises substantially solventlesspolyvinyl chloride matrix, fluorescent dyes and hindered amine lightstabilizers (all not shown) and is the outermost layer of article 70.

EXAMPLES

The following examples are provided to illustrate different embodimentsand details of the invention. Although the examples serve this purpose,the particular ingredients and amounts used as well as other conditionsand details are not to be construed in a manner that would unduly limitthe scope of this invention. Unless otherwise specified, all percentagesare in weight percent.

Accelerated Weathering

To simulate outdoor exposure to sunlight on an accelerated basis, somesamples were exposed to accelerated weathering in accordance with acycle defined by ASTM G-26 Type B, Method A. The light source was a6500-watt, water-cooled xenon arc device that has borosilicate inner andouter filters. The light source exhibits an irradiance of about 0.55watts/meter². The weathering cycle consisted of 102 minutes of light ata Black Panel temperature (as defined in the test method) of about 63°C., followed by 18 minutes of exposure while subjecting the sample todeionized water spray.

Ultraviolet-Visible (UV-Vis) Absorption Spectroscopy

The amount of fluorescent dye retained in a sample was determined bymeasuring the major dye absorption band (456 nanometers (nm)) usingUV-Vis spectroscopy before and after the sample was subjected toweathering. An illustrative UV-Vis spectrophotometer used was a Shimadzumodel UV22101-PC.

Following Beer's Law, a decrease in absorbance is related to a reductionin dye concentration. A “percent dye retention” value was calculated asthe ratio of the peak absorbance in the weathered sample to the peakabsorbance of the original unweathered sample.

The following abbreviations are used in the examples: Abbre- viationsMeaning PVC Polyvinyl chloride host matrix T-770 Hindered amine lightstabilizer TINUVIN ® 770 Bis-(2,2,6,6-tetramethyl-4-piperidinyl)sebacate Molecular weight of about 480 grams/mole Available fromCiba-Geigy Corp., Hawthorne, NY. T-144 Hindered amine light stabilizerTINUVIN ® 144Bis-(1,2,2,6,6-pentamethyl-4-piperidinyl)-2-n-butyl-2-(3,5-di-tert-butyl-4-hydroxybenzyl)malonate Molecular weight of about 685grams/mole Available from Ciba-Geigy Corp. PR-31 Hindered amine lightstabilizer Propanedioic acid, [(4-methoxyphenyl)-methylene]-bis-(1,2,2,6,6-pentamethyl-4-piperidinyl)ester Molecular weight of about 529grams/mole Available from Clariant Corp., Charlotte, NC. T-622 Hinderedamine light stabilizer TINUVIN ® 622 Dimethyl succinate polymer with4-hydroxy-2,2,6,6-tetramethyl- 1-piperidine ethanol Molecular weight(M_(n)) approximately greater than 2,500 grams/mole Available fromCiba-Geigy Corp. C-944 Hindered amine light stabilizer CHIMASORB ® 944FLPoly [6-[(1,1,3,3-tetramethylbutyl)amino]-s-triazine-2,4-diyl][2,2,6,6-tetramethyl-4-piperidyl)imino]hexamethylene[(2,2,6,6-tetramehtyl-4-piperidyl)imino)] Molecular weight (M_(n))approximately greater than 2,500 grams/mole Available from Ciba-GeigyCorp. T-440 Hindered amine light stabilizer TINUVIN ® 440 Low molecularweight acetylated hindered amine Molecular weight of about 435grams/mole Available from Ciba-Geigy Corp. C-3346 Hindered amine lightstabilizer CYASORB ® 3346 Oligomeric hindered amine Molecular weight(M_(n)) approximately greater than 1,600 grams/mole Available fromAmerican Cyanamid Corp. SO63 Thioxanthene orange fluorescent dyeHOSTASOL ® RED GG; 14H-anthra[2,1,9-mna]thioxanthene-14-one; Availablefrom Hoechst Celanese, Charlotte, NC. SY98 Thioxanthene yellowfluorescent dye HOSTASOL ® YELLOW 3GN-Octadecyl-benzo[k,1]thioxanthene-3,4-dicarboximide Available fromHoechst Celanese. D-304 Thioxanthene yellow fluorescent dye DAY-GLO ®304 Available from Day-Glo Color Corp., Cleveland, OH. D-315Thioxanthene orange fluorescent dye DAY-GLO ® 315 Available from Day-GloColor Corp. D-838 Coumarin fluorescent POTOMAC YELLOW ™ D-838 dyeAvailable from Day-Glo Color Corp. RED Anthrapyridone fluorescent reddye FLUORESCENT RED FB FB ™ Available from Keystone Aniline Corp.,Chicago, IL. RED Thioindigoid fluorescent red dye HOSTASOL ® RED 5B 5BC.I. (color index) Vat Red 41 Available from Hoechst Celanese.Abbreviations Meaning

-   PVC Polyvinyl chloride host matrix-   T-770 Hindered amine light stabilizer TINUVIN® 770    -   Bis-(2,2,6,6-tetramethyl-4-piperidinyl)sebacate    -   Molecular weight of about 480 grams/mole    -   Available from Ciba-Geigy Corp., Hawthorne, N.Y.-   T-144 Hindered amine light stabilizer TINUVIN® 144    -   Bis-(1,2,2,6,6-pentamethyl-4-piperidinyl)-2-n-butyl-2-(3,        5-di-tert-butyl-4-hydroxybenzyl)malonate    -   Molecular weight of about 685 grams/mole    -   Available from Ciba-Geigy Corp.-   PR-3 1 Hindered amine light stabilizer    -   Propanedioic acid,        [(4-methoxyphenyl)-methylene]-bis-(1,2,2,6,6-pentamethyl-4-piperidinyl)ester    -   Molecular weight of about 529 grams/mole    -   Available from Clariant Corp., Charlotte, N.C.-   T-622 Hindered amine light stabilizer TINUVIN® 622    -   Dimethyl succinate polymer with        4-hydroxy-2,2,6,6-tetramethyl-1-piperidine ethanol    -   Molecular weight (M_(n)) approximately greater than 2,500        grams/mole    -   Available from Ciba-Geigy Corp.-   C-944 Hindered amine light stabilizer CHIMASORB® 944FL    -   Poly        [6-[(1,1,3,3-tetramethylbutyl)amino]-s-triazine-2,4-diyl][2,2,6,6-tetramethyl-4-piperidyl)imino]hexamethylene[(2,2,6,6-tetramethyl-4-piperidyl)imino)]    -   Molecular weight (M_(n)) approximately greater than 2,500        grams/mole    -   Available from Ciba-Geigy Corp.-   T-440 Hindered amine light stabilizer TINUVIN® 440    -   Low molecular weight acetylated hindered amine    -   Molecular weight of about 435 grams/mole    -   Available from Ciba-Geigy Corp.-   C-3346 Hindered amine light stabilizer CYASORB® 3346    -   Oligomeric hindered amine    -   Molecular weight (M_(n)) approximately greater than 1,600        grams/mole    -   Available from American Cyanamid Corp.-   SO63 Thioxanthene orange fluorescent dye HOSTASOL® RED GG;    -   14H-anthra[2,1,9-mna]thioxanthene-14-one;    -   Available from Hoechst Celanese, Charlotte, N.C.-   SY98 Thioxanthene yellow fluorescent dye HOSTASOL® YELLOW 3G    -   N-Octadecyl-benzo[k,1]thioxanthene-3,4-dicarboximide    -   Available from Hoechst Celanese.-   D-304 Thioxanthene yellow fluorescent dye DAY-GLO° 304    -   Available from Day-Glo Color Corp., Cleveland, Ohio.-   D-315 Thioxanthene orange fluorescent dye DAY-GLO® 315    -   Available from Day-Glo Color Corp.-   D-838 Coumarin fluorescent POTOMAC YELLOW™ D-838 dye    -   Available from Day-Glo Color Corp.-   RED FB Anthrapyridone fluorescent red dye FLUORESCENT RED FB™    -   Available from Keystone Aniline Corp., Chicago, Ill.-   RED 5B Thioindigoid fluorescent red dye HOSTASOL® RED 5B    -   C.I. (color index) Vat Red 41    -   Available from Hoechst Celanese.

Example 1

A polyvinyl chloride film having a thickness of about 0.089 mm (0.0035inch) to about 0.11 mm (0.0045 inch) was made as follows. PVC resin(formulation S00354 containing UV absorbers from Alpha Chemical andPlastics Corp.) was mixed with about 0.2% SO63 fluorescent dye and about0.5% of the T-770 HALS. The resin/dye/HALS mixture was tumbled.mixed. Itwas then extruded into a substantially flat film using a single screwextruder with 5 heating zones set at about 175, 205, 205, 175 and 175°C. and the film die set at about 180° C. The extruder was athree-quarters (¾) inch single screw Brabender extruder with polishedchrome rolls.

The sample was subjected to 100 hours of weathering, and the data arereported in Tables 1 and 2.

Examples 2 and 3, and Comparative Examples A to B are all made accordingto Example 1 with different HALS used or no HALS used as described inTable 1. The samples were subjected to 100 hours of acceleratedweathering, and the data are reported in Table 1. TABLE 1 EXTRUDED PVCFILMS CONTAINING SO63 FLUORESCENT DYE WITH VARIOUS HALS Percent DyeRetention After 100 hours Example No. HALS weathering 1 T-770 55 2 T-14466 3 PR-31 61 Comparative A T-622 9 Comparative B C-944 14 Comparative CT-440 11 Comparative D C-3346 15 Comparative E None 7

As can be seen from the results of TABLE 1, a sample without any HALS(Comparative E) performed worst in that nearly all of the dye wasdepleted from the film. HALS that had a molecular weight exceeding 1000grams/mole (Comparative A, B and D) did poorly in the stabilization ofthe fluorescent dye. Comparative C, having a molecular weight of 435grams/mole, did not perform well because it did not contain at least onesecondary or tertiary amine group.

Examples 4 to 6 were made according to Example 1 except that differentfluorescent dyes were used as shown in Table 2. Unless otherwisespecified, the samples were subjected to 100 hours of acceleratedweathering, and the data are reported in Table 2.

Comparative Examples E to N were made according to Example 1 butdifferent fluorescent dyes were used with and without HALS as shown inTable 2. Unless otherwise specified, the samples were subjected to 100hours of accelerated weathering, and the data are reported in Table 2.TABLE 2 EXTRUDED PVC FILMS CONTAINING VARIOUS FLUORESCENT DYES WITH ANDWITHOUT HALS Percent Dye Example No. Fluorescent Dye HALS Used Retention1 SO63 T-770 55 4^(a) SY98 T-770 56 5 D-304 T-770 53 6^(a) D-315 T-77039 Comparative E SO63 None 7 Comparative F^(a) SY98 None 37 ComparativeG D-304 None 35 Comparative H^(a) D-315 None 17 Comparative I D-838T-770 9 Comparative J RED FB T-770 13 Comparative K^(b) RED 5B T-770 5Comparative L D-838 None 18 Comparative M RED FB None 25 ComparativeN^(b) RED 5B None 3^(a)Sample was subjected to 200 hours of accelerated weathering.^(b)Sample was subjected to 50 hours of accelerated weathering.

As shown in Table 2, samples of the invention containing thioxanthenefluorescent dyes with T-770 HALS (Examples 1, 4, 5 and 6) outperformedthose samples that did not contain thioxanthene fluorescent dyesstabilized with the same T-770 HALS (Comparatives I, J and K). Thosesamples that did contain thioxanthene fluorescent dyes but no HALS(Comparatives E, F, G and H) did not retain the dye as well as thosethat did contain HALS (Examples 1, 4, 5 and 6). Finally, comparingComparatives I, J and K with Comparatives L, M and N shows thatnon-thioxanthene fluorescent dye samples do not retain their color evenif HALS was used. Thus, in this situation, use of HALS, even if it isthe preferred HALS, was ineffective.

Example 7

A polyvinyl chloride film was made using a pilot scale calenderingprocess as follows. A powder of PVC was mixed with about 0.2% SY98fluorescent dye and about 0.5% T-770 HALS. Other additives, for exampleUV absorbers, heat stabilizers, plasticizers, lubricants, and fillerswere added either for processing aid or to help make a flexible PVCfilm. The mixture was fed through a strainer to remove metal, ifpresent. The mixture was continuously mixed to form a fused mass,milled, and fed through rolls, all heated at about 177° C. (350° F.), toform the inventive film about 0.13 mm to about 0.15 mm (0.005 to 0.006inch) thick. The sample was subjected to 400 hours of acceleratedweathering and the data are reported in Table 3 below.

Comparative O

A calendered PVC film was made according to Example 7 except that noHALS was added to the PVC powder. The sample was subjected to 400 hoursof accelerated weathering, and the data are reported in Table 3. TABLE 3CALENDERED PVC FILMS CONTAINING SY98 FLUORESCENT DYE Percent DyeRetention Example No. HALS Fluorescent Dye (After 400 hours) 7 T-770SY98 76 Comparative O None SY98 1.3

As shown in Table 3, calendered PVC film of the invention containing afluorescent dye and a HALS clearly outperformed a sample that did notcontain a HALS.

Example 4 and its comparative counterpart, Comparative E, were bothexposed to 400 hours of accelerated weathering, and the data arereported in Table 4. TABLE 4 EXTRUDED PVC FILMS CONTAINING SY98FLUORESCENT DYE Percent Dye Retention Example No. HALS Fluorescent Dye(After 400 hours) 4 T-770 SY98 29 Comparative E None SY98 9

As shown in Table 4, extruded PVC film of the invention containing afluorescent dye and a HALS outperformed a sample that did not contain aHALS. All references cited herein are incorporated by reference in eachreference's entirety.

1-23. (canceled)
 24. A fluorescence-stabilized polymeric compositioncomprising: a) polyvinyl chloride; b) a fluorescent dye; c) anultraviolet absorber; and d) a hindered amine light stabilizer compound.25. A fluorescence-stabilized polymeric composition of claim 24 whereinsaid fluorescent dye is a thioxanthene.
 26. A fluorescence-stabilizedpolymeric composition of claim 24 wherein said ultraviolet absorber is abenzophenone.
 27. A retroreflective structure comprising the polymericcomposition of claim 24.